Tuned only vehicles in arcade mode may be just so that the game is balanced. It may have been too easy just to tune cars to max and win arcade easily. Maybe its done this way to make arcade more structured.
Also the very term "arcade" should be a simpliefied version. Therefore having vehicles without tuning leaves this part of the game as an arcade rather than simulation version.
I'm very happy that long races can now be set in arcade mode, where I believe you can now set laps up to 50.
50 laps round the ring anyone?